This invention relates to a hydraulic flow metering circuit and a metering valve therefor and particularly to such circuits and valves having multiple functions.
In the control of return fluid flow from loads applied to hydraulic actuators such as cylinders on earthmoving and construction vehicles, it is customary to provide spool-type valve elements with each valve element serving various individual functions such as speed control of lowering loads, limiting excessive pressure, preventing or minimizing cavitation in the hydraulic cylinders, and holding a load stationary in a preselected position.
However, use of spool-type valve elements having a movable spool member lead to certain disadvantages, such as instability of the spool member while throttling fluid flow to decelerate a lowering load resulting in erratic or jerking movements of the load, and drifting of a stationary load from a preselected position due to leakage of fluid past clearances necessary to proper operation of the spool member.
The present invention is directed to a circuit comprising a single hydraulic flow metering valve which will function as a proportional speed control valve for lowering loads, a pressure limiting relief valve, and an anti-cavitation check valve.
In accordance with the present invention, a poppet type main valve meters return fluid flow from loaded cylinders in response to a variable orifice formed in the main valve, which is controlled by an integral servo valve, and to fluid flow through a bleed flow orifice which is controlled by a pilot valve. The metering valve is designed so that the following three different valve functions can be accomplished:
When the main valve is controlled by fluid flow through the variable orifice, the servo valve provides a machine operator with a proportional speed control for lowering loads.
If cylinder fluid pressure drops below return fluid pressure in the system, the main valve will open allowing fluid from the return to flow to the cylinder thereby operating as an anti-cavitation check valve.
The pilot valve controls bleed flow from the bleed flow orifice through the main valve to provide operation as a pressure limiting relief valve.
A fuller understanding of the invention may be had from consideration of the following description and claims taken together with the accompanying drawings.